Preheat the oven to 200ºC/fan 180ºC/gas mark 6. Put the sausage meat in a bowl with the tomatoes, spring onions, parsley, lemon zest, breadcrumbs and a good grinding of black pepper. Mix together with a wooden spoon, or with your hands, until well combined.
Cut the pastry block in half and, on a lightly floured surface, roll into a long rectangle shape, measuring about 40cm x 15cm, with a thickness of about 3mm. Form half the filling into a log about the length of the pastry. Put the sausage log on the pastry about 5mm in from the edge.
Brush the edge with the beaten egg, then fold the pastry over the sausage filling and press to seal the edge. Crimp using your fingers or press down with a fork, then cut into 6 rolls. Prepare a second batch using the remaining pastry and filling.
Mix together the sesame seeds and poppy seeds. Using a sharp knife, make a slit in the top of each sausage roll, then brush with beaten egg and sprinkle with the seeds. Bake for 20 minutes until golden and risen. Remove from the oven and cool slightly on a wire rack.